/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */
package com.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.base.Predicates.equalTo;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.instanceOf;
import static com.google.common.base.Predicates.not;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static com.google.common.util.concurrent.Service.State.FAILED;
import static com.google.common.util.concurrent.Service.State.NEW;
import static com.google.common.util.concurrent.Service.State.RUNNING;
import static com.google.common.util.concurrent.Service.State.STARTING;
import static com.google.common.util.concurrent.Service.State.STOPPING;
import static com.google.common.util.concurrent.Service.State.TERMINATED;
import static java.util.concurrent.TimeUnit.MILLISECONDS;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.MoreObjects;
import com.google.common.base.Stopwatch;
import com.google.common.collect.Collections2;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableMultimap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ImmutableSetMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.MultimapBuilder;
import com.google.common.collect.Multimaps;
import com.google.common.collect.Multiset;
import com.google.common.collect.Ordering;
import com.google.common.collect.SetMultimap;
import com.google.common.util.concurrent.ListenerCallQueue.Callback;
import com.google.common.util.concurrent.Service.State;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.j2objc.annotations.WeakOuter;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collections;
import java.util.EnumSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.concurrent.GuardedBy;

/**
 * A manager for monitoring and controlling a set of {@linkplain Service services}. This class
 * provides methods for {@linkplain #startAsync() starting}, {@linkplain #stopAsync() stopping} and
 * {@linkplain #servicesByState inspecting} a collection of {@linkplain Service services}.
 * Additionally, users can monitor state transitions with the {@linkplain Listener listener}
 * mechanism.
 *
 * <p>
 * While it is recommended that service lifecycles be managed via this class, state transitions
 * initiated via other mechanisms do not impact the correctness of its methods. For example, if the
 * services are started by some mechanism besides {@link #startAsync}, the listeners will be invoked
 * when appropriate and {@link #awaitHealthy} will still work as expected.
 *
 * <p>
 * Here is a simple example of how to use a {@code ServiceManager} to start a server.
 * 
 * <pre>
 *    {@code
 * class Server {
 *   public static void main(String[] args) {
 *     Set<Service> services = ...;
 *     ServiceManager manager = new ServiceManager(services);
 *     manager.addListener(new Listener() {
 *         public void stopped() {}
 *         public void healthy() {
 *           // Services have been initialized and are healthy, start accepting requests...
 *         }
 *         public void failure(Service service) {
 *           // Something failed, at this point we could log it, notify a load balancer, or take
 *           // some other action.  For now we will just exit.
 *           System.exit(1);
 *         }
 *       },
 *       MoreExecutors.directExecutor());
 *
 *     Runtime.getRuntime().addShutdownHook(new Thread() {
 *       public void run() {
 *         // Give the services 5 seconds to stop to ensure that we are responsive to shutdown
 *         // requests.
 *         try {
 *           manager.stopAsync().awaitStopped(5, TimeUnit.SECONDS);
 *         } catch (TimeoutException timeout) {
 *           // stopping timed out
 *         }
 *       }
 *     });
 *     manager.startAsync();  // start all the services asynchronously
 *   }
 * }}
 * </pre>
 *
 * <p>
 * This class uses the ServiceManager's methods to start all of its services, to respond to service
 * failure and to ensure that when the JVM is shutting down all the services are stopped.
 *
 * @author Luke Sandberg
 * @since 14.0
 */
@Beta
@GwtIncompatible
public final class ServiceManager {
    private static final Logger logger = Logger.getLogger(ServiceManager.class.getName());
    private static final Callback<Listener> HEALTHY_CALLBACK = new Callback<Listener>("healthy()") {
        @Override
        void call(Listener listener) {
            listener.healthy();
        }
    };
    private static final Callback<Listener> STOPPED_CALLBACK = new Callback<Listener>("stopped()") {
        @Override
        void call(Listener listener) {
            listener.stopped();
        }
    };

    /**
     * A listener for the aggregate state changes of the services that are under management. Users
     * that need to listen to more fine-grained events (such as when each particular
     * {@linkplain Service service} starts, or terminates), should attach
     * {@linkplain Service.Listener service listeners} to each individual service.
     *
     * @author Luke Sandberg
     * @since 15.0 (present as an interface in 14.0)
     */
    @Beta // Should come out of Beta when ServiceManager does
    public abstract static class Listener {
        /**
         * Called when the service initially becomes healthy.
         *
         * <p>
         * This will be called at most once after all the services have entered the
         * {@linkplain State#RUNNING running} state. If any services fail during start up or
         * {@linkplain State#FAILED fail}/{@linkplain State#TERMINATED terminate} before all other
         * services have started {@linkplain State#RUNNING running} then this method will not be
         * called.
         */
        public void healthy() {}

        /**
         * Called when the all of the component services have reached a terminal state, either
         * {@linkplain State#TERMINATED terminated} or {@linkplain State#FAILED failed}.
         */
        public void stopped() {}

        /**
         * Called when a component service has {@linkplain State#FAILED failed}.
         *
         * @param service The service that failed.
         */
        public void failure(Service service) {}
    }

    /**
     * An encapsulation of all of the state that is accessed by the {@linkplain ServiceListener
     * service listeners}. This is extracted into its own object so that {@link ServiceListener}
     * could be made {@code static} and its instances can be safely constructed and added in the
     * {@link ServiceManager} constructor without having to close over the partially constructed
     * {@link ServiceManager} instance (i.e. avoid leaking a pointer to {@code this}).
     */
    private final ServiceManagerState state;
    private final ImmutableList<Service> services;

    /**
     * Constructs a new instance for managing the given services.
     *
     * @param services The services to manage
     *
     * @throws IllegalArgumentException if not all services are {@linkplain State#NEW new} or if
     *         there are any duplicate services.
     */
    public ServiceManager(Iterable<? extends Service> services) {
        ImmutableList<Service> copy = ImmutableList.copyOf(services);
        if (copy.isEmpty()) {
            // Having no services causes the manager to behave strangely. Notably, listeners are
            // never
            // fired. To avoid this we substitute a placeholder service.
            logger.log(Level.WARNING,
                    "ServiceManager configured with no services.  Is your application configured properly?",
                    new EmptyServiceManagerWarning());
            copy = ImmutableList.<Service>of(new NoOpService());
        }
        this.state = new ServiceManagerState(copy);
        this.services = copy;
        WeakReference<ServiceManagerState> stateReference = new WeakReference<ServiceManagerState>(state);
        for (Service service : copy) {
            service.addListener(new ServiceListener(service, stateReference), directExecutor());
            // We check the state after adding the listener as a way to ensure that our listener was
            // added
            // to a NEW service.
            checkArgument(service.state() == NEW, "Can only manage NEW services, %s", service);
        }
        // We have installed all of our listeners and after this point any state transition should
        // be
        // correct.
        this.state.markReady();
    }

    /**
     * Registers a {@link Listener} to be {@linkplain Executor#execute executed} on the given
     * executor. The listener will not have previous state changes replayed, so it is suggested that
     * listeners are added before any of the managed services are {@linkplain Service#startAsync
     * started}.
     *
     * <p>
     * {@code addListener} guarantees execution ordering across calls to a given listener but not
     * across calls to multiple listeners. Specifically, a given listener will have its callbacks
     * invoked in the same order as the underlying service enters those states. Additionally, at
     * most one of the listener's callbacks will execute at once. However, multiple listeners'
     * callbacks may execute concurrently, and listeners may execute in an order different from the
     * one in which they were registered.
     *
     * <p>
     * RuntimeExceptions thrown by a listener will be caught and logged. Any exception thrown during
     * {@code Executor.execute} (e.g., a {@code RejectedExecutionException}) will be caught and
     * logged.
     *
     * <p>
     * For fast, lightweight listeners that would be safe to execute in any thread, consider calling
     * {@link #addListener(Listener)}.
     *
     * @param listener the listener to run when the manager changes state
     * @param executor the executor in which the listeners callback methods will be run.
     */
    public void addListener(Listener listener, Executor executor) {
        state.addListener(listener, executor);
    }

    /**
     * Registers a {@link Listener} to be run when this {@link ServiceManager} changes state. The
     * listener will not have previous state changes replayed, so it is suggested that listeners are
     * added before any of the managed services are {@linkplain Service#startAsync started}.
     *
     * <p>
     * {@code addListener} guarantees execution ordering across calls to a given listener but not
     * across calls to multiple listeners. Specifically, a given listener will have its callbacks
     * invoked in the same order as the underlying service enters those states. Additionally, at
     * most one of the listener's callbacks will execute at once. However, multiple listeners'
     * callbacks may execute concurrently, and listeners may execute in an order different from the
     * one in which they were registered.
     *
     * <p>
     * RuntimeExceptions thrown by a listener will be caught and logged.
     *
     * @param listener the listener to run when the manager changes state
     */
    public void addListener(Listener listener) {
        state.addListener(listener, directExecutor());
    }

    /**
     * Initiates service {@linkplain Service#startAsync startup} on all the services being managed.
     * It is only valid to call this method if all of the services are {@linkplain State#NEW new}.
     *
     * @return this
     * @throws IllegalStateException if any of the Services are not {@link State#NEW new} when the
     *         method is called.
     */
    @CanIgnoreReturnValue
    public ServiceManager startAsync() {
        for (Service service : services) {
            State state = service.state();
            checkState(state == NEW, "Service %s is %s, cannot start it.", service, state);
        }
        for (Service service : services) {
            try {
                state.tryStartTiming(service);
                service.startAsync();
            } catch (IllegalStateException e) {
                // This can happen if the service has already been started or stopped (e.g. by
                // another
                // service or listener). Our contract says it is safe to call this method if
                // all services were NEW when it was called, and this has already been verified
                // above, so we
                // don't propagate the exception.
                logger.log(Level.WARNING, "Unable to start Service " + service, e);
            }
        }
        return this;
    }

    /**
     * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy}. The manager
     * will become healthy after all the component services have reached the
     * {@linkplain State#RUNNING running} state.
     *
     * @throws IllegalStateException if the service manager reaches a state from which it cannot
     *         become {@linkplain #isHealthy() healthy}.
     */
    public void awaitHealthy() {
        state.awaitHealthy();
    }

    /**
     * Waits for the {@link ServiceManager} to become {@linkplain #isHealthy() healthy} for no more
     * than the given time. The manager will become healthy after all the component services have
     * reached the {@linkplain State#RUNNING running} state.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @throws TimeoutException if not all of the services have finished starting within the
     *         deadline
     * @throws IllegalStateException if the service manager reaches a state from which it cannot
     *         become {@linkplain #isHealthy() healthy}.
     */
    public void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
        state.awaitHealthy(timeout, unit);
    }

    /**
     * Initiates service {@linkplain Service#stopAsync shutdown} if necessary on all the services
     * being managed.
     *
     * @return this
     */
    @CanIgnoreReturnValue
    public ServiceManager stopAsync() {
        for (Service service : services) {
            service.stopAsync();
        }
        return this;
    }

    /**
     * Waits for the all the services to reach a terminal state. After this method returns all
     * services will either be {@linkplain Service.State#TERMINATED terminated} or
     * {@linkplain Service.State#FAILED failed}.
     */
    public void awaitStopped() {
        state.awaitStopped();
    }

    /**
     * Waits for the all the services to reach a terminal state for no more than the given time.
     * After this method returns all services will either be {@linkplain Service.State#TERMINATED
     * terminated} or {@linkplain Service.State#FAILED failed}.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @throws TimeoutException if not all of the services have stopped within the deadline
     */
    public void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
        state.awaitStopped(timeout, unit);
    }

    /**
     * Returns true if all services are currently in the {@linkplain State#RUNNING running} state.
     *
     * <p>
     * Users who want more detailed information should use the {@link #servicesByState} method to
     * get detailed information about which services are not running.
     */
    public boolean isHealthy() {
        for (Service service : services) {
            if (!service.isRunning()) {
                return false;
            }
        }
        return true;
    }

    /**
     * Provides a snapshot of the current state of all the services under management.
     *
     * <p>
     * N.B. This snapshot is guaranteed to be consistent, i.e. the set of states returned will
     * correspond to a point in time view of the services.
     */
    public ImmutableMultimap<State, Service> servicesByState() {
        return state.servicesByState();
    }

    /**
     * Returns the service load times. This value will only return startup times for services that
     * have finished starting.
     *
     * @return Map of services and their corresponding startup time in millis, the map entries will
     *         be ordered by startup time.
     */
    public ImmutableMap<Service, Long> startupTimes() {
        return state.startupTimes();
    }

    @Override
    public String toString() {
        return MoreObjects.toStringHelper(ServiceManager.class)
                .add("services", Collections2.filter(services, not(instanceOf(NoOpService.class)))).toString();
    }

    /**
     * An encapsulation of all the mutable state of the {@link ServiceManager} that needs to be
     * accessed by instances of {@link ServiceListener}.
     */
    private static final class ServiceManagerState {
        final Monitor monitor = new Monitor();

        @GuardedBy("monitor")
        final SetMultimap<State, Service> servicesByState =
                MultimapBuilder.enumKeys(State.class).linkedHashSetValues().build();

        @GuardedBy("monitor")
        final Multiset<State> states = servicesByState.keys();

        @GuardedBy("monitor")
        final Map<Service, Stopwatch> startupTimers = Maps.newIdentityHashMap();

        /**
         * These two booleans are used to mark the state as ready to start.
         *
         * <p>
         * {@link #ready}: is set by {@link #markReady} to indicate that all listeners have been
         * correctly installed
         *
         * <p>
         * {@link #transitioned}: is set by {@link #transitionService} to indicate that some
         * transition has been performed.
         *
         * <p>
         * Together, they allow us to enforce that all services have their listeners installed prior
         * to any service performing a transition, then we can fail in the ServiceManager
         * constructor rather than in a Service.Listener callback.
         */
        @GuardedBy("monitor")
        boolean ready;

        @GuardedBy("monitor")
        boolean transitioned;

        final int numberOfServices;

        /**
         * Controls how long to wait for all the services to either become healthy or reach a state
         * from which it is guaranteed that it can never become healthy.
         */
        final Monitor.Guard awaitHealthGuard = new AwaitHealthGuard();

        @WeakOuter
        final class AwaitHealthGuard extends Monitor.Guard {
            AwaitHealthGuard() {
                super(ServiceManagerState.this.monitor);
            }

            @Override
            public boolean isSatisfied() {
                // All services have started or some service has terminated/failed.
                return states.count(RUNNING) == numberOfServices || states.contains(STOPPING)
                        || states.contains(TERMINATED) || states.contains(FAILED);
            }
        }

        /**
         * Controls how long to wait for all services to reach a terminal state.
         */
        final Monitor.Guard stoppedGuard = new StoppedGuard();

        @WeakOuter
        final class StoppedGuard extends Monitor.Guard {
            StoppedGuard() {
                super(ServiceManagerState.this.monitor);
            }

            @Override
            public boolean isSatisfied() {
                return states.count(TERMINATED) + states.count(FAILED) == numberOfServices;
            }
        }

        /** The listeners to notify during a state transition. */
        @GuardedBy("monitor")
        final List<ListenerCallQueue<Listener>> listeners =
                Collections.synchronizedList(new ArrayList<ListenerCallQueue<Listener>>());

        /**
         * It is implicitly assumed that all the services are NEW and that they will all remain NEW
         * until all the Listeners are installed and {@link #markReady()} is called. It is our
         * caller's responsibility to only call {@link #markReady()} if all services were new at the
         * time this method was called and when all the listeners were installed.
         */
        ServiceManagerState(ImmutableCollection<Service> services) {
            this.numberOfServices = services.size();
            servicesByState.putAll(NEW, services);
        }

        /**
         * Attempts to start the timer immediately prior to the service being started via
         * {@link Service#startAsync()}.
         */
        void tryStartTiming(Service service) {
            monitor.enter();
            try {
                Stopwatch stopwatch = startupTimers.get(service);
                if (stopwatch == null) {
                    startupTimers.put(service, Stopwatch.createStarted());
                }
            } finally {
                monitor.leave();
            }
        }

        /**
         * Marks the {@link State} as ready to receive transitions. Returns true if no transitions
         * have been observed yet.
         */
        void markReady() {
            monitor.enter();
            try {
                if (!transitioned) {
                    // nothing has transitioned since construction, good.
                    ready = true;
                } else {
                    // This should be an extremely rare race condition.
                    List<Service> servicesInBadStates = Lists.newArrayList();
                    for (Service service : servicesByState().values()) {
                        if (service.state() != NEW) {
                            servicesInBadStates.add(service);
                        }
                    }
                    throw new IllegalArgumentException("Services started transitioning asynchronously before "
                            + "the ServiceManager was constructed: " + servicesInBadStates);
                }
            } finally {
                monitor.leave();
            }
        }

        void addListener(Listener listener, Executor executor) {
            checkNotNull(listener, "listener");
            checkNotNull(executor, "executor");
            monitor.enter();
            try {
                // no point in adding a listener that will never be called
                if (!stoppedGuard.isSatisfied()) {
                    listeners.add(new ListenerCallQueue<Listener>(listener, executor));
                }
            } finally {
                monitor.leave();
            }
        }

        void awaitHealthy() {
            monitor.enterWhenUninterruptibly(awaitHealthGuard);
            try {
                checkHealthy();
            } finally {
                monitor.leave();
            }
        }

        void awaitHealthy(long timeout, TimeUnit unit) throws TimeoutException {
            monitor.enter();
            try {
                if (!monitor.waitForUninterruptibly(awaitHealthGuard, timeout, unit)) {
                    throw new TimeoutException("Timeout waiting for the services to become healthy. The "
                            + "following services have not started: "
                            + Multimaps.filterKeys(servicesByState, in(ImmutableSet.of(NEW, STARTING))));
                }
                checkHealthy();
            } finally {
                monitor.leave();
            }
        }

        void awaitStopped() {
            monitor.enterWhenUninterruptibly(stoppedGuard);
            monitor.leave();
        }

        void awaitStopped(long timeout, TimeUnit unit) throws TimeoutException {
            monitor.enter();
            try {
                if (!monitor.waitForUninterruptibly(stoppedGuard, timeout, unit)) {
                    throw new TimeoutException(
                            "Timeout waiting for the services to stop. The following " + "services have not stopped: "
                                    + Multimaps.filterKeys(servicesByState, not(in(EnumSet.of(TERMINATED, FAILED)))));
                }
            } finally {
                monitor.leave();
            }
        }

        ImmutableMultimap<State, Service> servicesByState() {
            ImmutableSetMultimap.Builder<State, Service> builder = ImmutableSetMultimap.builder();
            monitor.enter();
            try {
                for (Entry<State, Service> entry : servicesByState.entries()) {
                    if (!(entry.getValue() instanceof NoOpService)) {
                        builder.put(entry);
                    }
                }
            } finally {
                monitor.leave();
            }
            return builder.build();
        }

        ImmutableMap<Service, Long> startupTimes() {
            List<Entry<Service, Long>> loadTimes;
            monitor.enter();
            try {
                loadTimes = Lists.newArrayListWithCapacity(startupTimers.size());
                // N.B. There will only be an entry in the map if the service has started
                for (Entry<Service, Stopwatch> entry : startupTimers.entrySet()) {
                    Service service = entry.getKey();
                    Stopwatch stopWatch = entry.getValue();
                    if (!stopWatch.isRunning() && !(service instanceof NoOpService)) {
                        loadTimes.add(Maps.immutableEntry(service, stopWatch.elapsed(MILLISECONDS)));
                    }
                }
            } finally {
                monitor.leave();
            }
            Collections.sort(loadTimes, Ordering.natural().onResultOf(new Function<Entry<Service, Long>, Long>() {
                @Override
                public Long apply(Map.Entry<Service, Long> input) {
                    return input.getValue();
                }
            }));
            return ImmutableMap.copyOf(loadTimes);
        }

        /**
         * Updates the state with the given service transition.
         *
         * <p>
         * This method performs the main logic of ServiceManager in the following steps.
         * <ol>
         * <li>Update the {@link #servicesByState()}
         * <li>Update the {@link #startupTimers}
         * <li>Based on the new state queue listeners to run
         * <li>Run the listeners (outside of the lock)
         * </ol>
         */
        void transitionService(final Service service, State from, State to) {
            checkNotNull(service);
            checkArgument(from != to);
            monitor.enter();
            try {
                transitioned = true;
                if (!ready) {
                    return;
                }
                // Update state.
                checkState(servicesByState.remove(from, service),
                        "Service %s not at the expected location in the state map %s", service, from);
                checkState(servicesByState.put(to, service), "Service %s in the state map unexpectedly at %s", service,
                        to);
                // Update the timer
                Stopwatch stopwatch = startupTimers.get(service);
                if (stopwatch == null) {
                    // This means the service was started by some means other than
                    // ServiceManager.startAsync
                    stopwatch = Stopwatch.createStarted();
                    startupTimers.put(service, stopwatch);
                }
                if (to.compareTo(RUNNING) >= 0 && stopwatch.isRunning()) {
                    // N.B. if we miss the STARTING event then we may never record a startup time.
                    stopwatch.stop();
                    if (!(service instanceof NoOpService)) {
                        logger.log(Level.FINE, "Started {0} in {1}.", new Object[] {service, stopwatch});
                    }
                }
                // Queue our listeners

                // Did a service fail?
                if (to == FAILED) {
                    fireFailedListeners(service);
                }

                if (states.count(RUNNING) == numberOfServices) {
                    // This means that the manager is currently healthy. N.B. If other threads call
                    // isHealthy
                    // they are not guaranteed to get 'true', because any service could fail right
                    // now.
                    fireHealthyListeners();
                } else if (states.count(TERMINATED) + states.count(FAILED) == numberOfServices) {
                    fireStoppedListeners();
                }
            } finally {
                monitor.leave();
                // Run our executors outside of the lock
                executeListeners();
            }
        }

        @GuardedBy("monitor")
        void fireStoppedListeners() {
            STOPPED_CALLBACK.enqueueOn(listeners);
        }

        @GuardedBy("monitor")
        void fireHealthyListeners() {
            HEALTHY_CALLBACK.enqueueOn(listeners);
        }

        @GuardedBy("monitor")
        void fireFailedListeners(final Service service) {
            new Callback<Listener>("failed({service=" + service + "})") {
                @Override
                void call(Listener listener) {
                    listener.failure(service);
                }
            }.enqueueOn(listeners);
        }

        /** Attempts to execute all the listeners in {@link #listeners}. */
        void executeListeners() {
            checkState(!monitor.isOccupiedByCurrentThread(),
                    "It is incorrect to execute listeners with the monitor held.");
            // iterate by index to avoid concurrent modification exceptions
            for (int i = 0; i < listeners.size(); i++) {
                listeners.get(i).execute();
            }
        }

        @GuardedBy("monitor")
        void checkHealthy() {
            if (states.count(RUNNING) != numberOfServices) {
                IllegalStateException exception = new IllegalStateException(
                        "Expected to be healthy after starting. The following services are not running: "
                                + Multimaps.filterKeys(servicesByState, not(equalTo(RUNNING))));
                throw exception;
            }
        }
    }

    /**
     * A {@link Service} that wraps another service and times how long it takes for it to start and
     * also calls the {@link ServiceManagerState#transitionService(Service, State, State)}, to
     * record the state transitions.
     */
    private static final class ServiceListener extends Service.Listener {
        final Service service;
        // We store the state in a weak reference to ensure that if something went wrong while
        // constructing the ServiceManager we don't pointlessly keep updating the state.
        final WeakReference<ServiceManagerState> state;

        ServiceListener(Service service, WeakReference<ServiceManagerState> state) {
            this.service = service;
            this.state = state;
        }

        @Override
        public void starting() {
            ServiceManagerState state = this.state.get();
            if (state != null) {
                state.transitionService(service, NEW, STARTING);
                if (!(service instanceof NoOpService)) {
                    logger.log(Level.FINE, "Starting {0}.", service);
                }
            }
        }

        @Override
        public void running() {
            ServiceManagerState state = this.state.get();
            if (state != null) {
                state.transitionService(service, STARTING, RUNNING);
            }
        }

        @Override
        public void stopping(State from) {
            ServiceManagerState state = this.state.get();
            if (state != null) {
                state.transitionService(service, from, STOPPING);
            }
        }

        @Override
        public void terminated(State from) {
            ServiceManagerState state = this.state.get();
            if (state != null) {
                if (!(service instanceof NoOpService)) {
                    logger.log(Level.FINE, "Service {0} has terminated. Previous state was: {1}",
                            new Object[] {service, from});
                }
                state.transitionService(service, from, TERMINATED);
            }
        }

        @Override
        public void failed(State from, Throwable failure) {
            ServiceManagerState state = this.state.get();
            if (state != null) {
                // Log before the transition, so that if the process exits in response to server
                // failure,
                // there is a higher likelihood that the cause will be in the logs.
                boolean log = !(service instanceof NoOpService);
                if (log) {
                    logger.log(Level.SEVERE, "Service " + service + " has failed in the " + from + " state.", failure);
                }
                state.transitionService(service, from, FAILED);
            }
        }
    }

    /**
     * A {@link Service} instance that does nothing. This is only useful as a placeholder to ensure
     * that the {@link ServiceManager} functions properly even when it is managing no services.
     *
     * <p>
     * The use of this class is considered an implementation detail of ServiceManager and as such it
     * is excluded from {@link #servicesByState}, {@link #startupTimes}, {@link #toString} and all
     * logging statements.
     */
    private static final class NoOpService extends AbstractService {
        @Override
        protected void doStart() {
            notifyStarted();
        }

        @Override
        protected void doStop() {
            notifyStopped();
        }
    }

    /** This is never thrown but only used for logging. */
    private static final class EmptyServiceManagerWarning extends Throwable {
    }
}
